This program has been directed at obtaining insights into how cell and gene therapy approaches can provide novel therapies for liver disorders. The general hypothesis is that transplanted liver cells will survive and function in the context of a permissive microenvironment, which is best provided by the liver. After cell transplantation, a variety of interactions between transplanted cells and the native liver are necessary for transplanted cells to engraft and proliferate. These interactions encompass other liver cell types and soluble factors released by various cell types that are normally resident in the liver or could enter the liver after cells have been transplanted. These mechanisms may alter whether transplanted cells can engraft, survive, function or proliferate in the liver. Cell and gene therapy requires development of suitable masses of transplanted cells for disease correction, although these requirements can vary from disease-to-disease. Therefore, a variety of mechanisms concerning engraftment and proliferation of transplanted cells in specific disorders must be studied in suitable animal models. Such studies need to include analysis of mature hepatocytes, as well as assays to assess the properties of stem/progenitor cells for obtaining appropriate clinical strategies. Progress in our laboratory over the past two decades has led to working models of transplanted cell engraftment and proliferation in the liver. Several important mechanisms have been identified that regulate survival of transplanted cells during the early period after cell transplantation, as well as after transplanted cells have become an integral part of the liver parenchyma. Specific perturbations in the host liver that promote cell engraftment include analysis of cell-cell interactions in liver sinusoids, modulations in the liver microenvironment and aspects of the biological properties of transplanted cells themselves. Genotoxic injury in native hepatocytes that offers competitive advantages for transplanted cells to begin proliferating constitutes an effective strategy for liver repopulation. Therefore, we propose a series of studies to further define transplanted cell engraftment in settings where the liver is either normal or diseased. We will study the fate of transplanted liver cells in rodents, including immunodeficient mice, to generate further animal models. We will determine whether transplanted cells will proliferate following further ways to induce liver injury in recipient animals prior to cell transplantation. We will use these principles to approach correction of acquired and genetic disorders in animals. Completion of our studies will generate new knowledge in liver repopulation and cell and gene therapy mechanisms. This will lead to more effective liver-directed cell and gene therapy in people.